1. Field of Art
The disclosure relates generally to percussion or hammer drilling assemblies for drilling applications, including those for recovery of oil and gas. More particularly, the disclosure relates to a feed tube disposed within a percussion drilling assembly for supplying pressurized fluid to reciprocate a piston, causing the piston to cyclically impact a drill bit that is coupled to the drilling assembly.
2. Background of Related Art
A percussion drilling assembly is typically coupled to the lower end of a rotatable drill string. The percussion drilling assembly includes a top sub coupled to the drill string, a driver sub which couples to a drill bit and a piston-cylinder assembly positioned therebetween. The piston-cylinder assembly includes a casing surrounding a piston that reciprocates within the casing. A feed tube assembly is suspended within the casing and supported by the top sub.
During operation of the percussion drilling assembly, the drill string rotates the percussion-drilling assembly with the drill bit coupled thereto. Pressurized fluid, such as compressed air or nitrogen, is delivered from the drill string through the feed tube to the upper and lower ends of the piston in an alternating fashion, causing the piston to reciprocate within the casing. When at its lowest position at the end of the downstroke, the piston impacts the drill bit, thereby causing the drill bit to impact the formation below the bit. As the drill bit alternately impacts and rotates against the formation, the drill bit crushes, breaks, and loosens formation material to create a borehole along a predetermined path toward a target zone where oil or gas, for example, may be recovered.
The coupling between the top sub and the feed tube is important to the function of the percussion drilling assembly for a number of reasons. The coupling enables the top sub to support the suspended feed tube. Examples of such couplings include a cross pin that is inserted through the lower end of the top sub and the upper end of the feed tube, such that the pin suspends the feed tube from the top sub. One limitation of this design is that the feed tube is rigidly attached to the top sub and any slight misalignments and errors in machining can result in side loading, which may lead to premature wear of the sliding components. In another design, the upper end of the feed tube includes a flange having an outer edge used to suspend the feed tube from a shoulder of the top sub. In this case, O-rings are used to provide limited flexibility to the arrangement, and additional complexity and components are required to suspend the feed tube, making it more difficult to service the hammer assembly. The lack of flexibility may exacerbate wear and thereby increase tolerances between mating components of the drilling assembly, such as the feed tube and the piston. In turn, increase in clearance between the mating components may result in decreased system efficiency.
The coupling between the top sub and the feed tube may also act as a seal. The top sub and casing are typically coupled by means of a threaded connection. Pressurized fluid supplied through the feed tube to reciprocate the piston occupies an upper chamber within the casing proximate the threaded connection. The coupling between the feed tube and the top sub may provide a seal that protects the threads from exposure to the pressurized fluid, and prevents loss of pressurized fluid through this connection. The absence of an effective seal at this location may result in loss of fluid pressure to reciprocate the piston, and thus loss of drilling efficiency, as well as corrosion to the threads, thereby reducing the service life of the percussion drilling assembly.
The coupling between the top sub and the feed tube also enables alignment of the feed tube within the piston. Misalignment of the feed tube within the piston reduces the efficiency of the percussion bit. Rigid couplings, such as a cross pin inserted through the top sub and feed tube, cannot accommodate for potential misalignment of the feed tube within the piston. Because of this, clearance between the feed tube and the piston is instead increased over the dimension that would otherwise be required to accommodate for potential misalignment of these components. Increasing the clearance between the feed tube and piston, however, reduces percussion bit efficiency due to increased leakage from the upper chamber to the lower pressure exhaust passage and increases manufacturing costs due to the additional machining required. Reduced efficiency, in turn, leads to a reduced rate of penetration (ROP) for the percussion drilling assembly, and thus increased drilling time and cost.
The length of time that a percussion drilling assembly may be employed before it must be changed depends upon its ROP and its durability. Increasing the ROP and the service life of the percussion drilling assembly will decrease drilling time and allow valuable oil and gas to be recovered more economically. Accordingly, feed tube assemblies that offer the potential to increase the ROP and the service life of the percussion drilling assembly would be particularly desirable.